Development of trichospores and their appendages in Genistellospora homothallica and other Harpellales and fine-structural evidence for the sporangial nature of trichospores

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Abstract
Stages in the formation of trichospores and their appendages and released and germinated trichospores of Genistellospora homothallica and other Harpellales (Trichomycetes) were studied by light and electron microscopy. The trichospore is shown to be a dehiscent, monosporous sporangium with the appendages contiguous with the sporangial wall. In G. homothallica the five–seven appendages develop within the generative cell at an early stage in trichospore formation after the fusion of vesicles containing appendage precursor material with specific sites on the plasmalemma. Each formed appendage is contained within an invagination of the plasmalemma, the appendage sac, and is surrounded by a matrix. Comparative studies with other genera of Harpellales indicate a similar mechanism of appendage formation but differences in their substructure, time of formation, and location within the generative cell. The significance of apical spore bodies and associated canals which penetrate the sporangiospore walls of mature trichospores is discussed in relation to spore germination and holdfast secretion.A hypothesis is presented for appendage formation, and two basic types of appendage arrangement within the generative cell are recognized. A comparison is made between the trichospore–generative cell complex of the Harpellales and the merosporangium–pseudophialide complex of the Kickxellales.